Beam handling during All-In-Vac operation
During the science mode, we want to operate solely on the PDs inside the vacuum. There will be no light coming out to the air at all. The main reason for this is to avoid scattering light problems (things in the air vibrate much more at audio frequencies due to accoustic noise). However, this configuration poses some challenges to us. First of all, all the light power has to be somehow dissipated in the vacuum. Even though the detection chambers (in which the PDs are installed) are at room temperature, we have to make sure that the heat is properly disposed. If the temperature of the PDs change too much during a lock, we may have slow drift problems. Secondly, beams from the interferometer are extracted out to the air and detected by large range (but more noisy) PDs for lock acquisition. Once the interferometer is locked, we have to close those windows. Just closing the lid is not enough because the lid will generate a heck of scattered light. We have to come up with a way to gracefully shut down the beam going out to the air. Moreover, just damping the beam is not enough at some detection ports. In order to maximize the light power going to the in-vac PDs (to minimize the shot noise), we may want to change the power ratio between the in-air and the in-vac PDs after the lock acquisition. We probably send more power to the air during the lock acquisition, while we want send most of the light to the in-vac PDs after the lock.
Task list
- Develop a way to remotely close the optical window without generating scattered light.
- Develop a way to remotely change the power ratio between the beam going to the air and the in-vac PDs.
Solutions
- For task 1, simply use pick-off mirrors to obstacle the beam path to the optical windows.
- For task 2, simply use diaphragms in front of PDs.
TODOs
- Find the appropriate products. Need vacuum compatibility.
- Clarify the locations of these gimmicks in the vacuum chambers.
- List up the required wires.